Dallas Semiconductors (now owned by Maxim) used to make a part in small two pin package that contained a unique 64 bit serial number (eg every one is different). I think it was the DS2400 and they were intended for applications like MAC addresses on NIC's.
One of these epoxied into a hole in the PCB would be impossible to remove and replace without damage.
The nearest I could find on the Maxim web site now are these ibuttons which are a bit too big I think but perhaps I didn't look hard enough.
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